These are all good points and I want to add to the interpretations.
You can chalk those 5 as individuals who Kaido consider to have been great at piracy (even Oden was a pirate too but also the only one to scar him). The reason Big Mom is not there is either because Kaido has no respect for her or better yet because they are currently allied so it would not make sense to show. BM is a pirate who has always been a prominent figure even before Kaido was an apprentice. Furthermore the individuals shown are only pirates and not marines or any other opponent Kaido has fought and been defeated/captured by.
For the second interpretation we could observe how Oda placed the characters. Besides it looking like a pyramid, I think there is an actual, chronological order that Kaido challenged these individuals and fought over his lifetime… and it goes clockwise.
1) His Captain, Xebec (before joining rocks pirates?)
2) Roger (during god valley)
3) Whitebeard (during god valley or some time after it)
4) Oden (25 and 20 years ago)
5) Shanks (sometime when he became an emperor likely and/or two years ago)
6) Luffy (the present, and the person who should ideally stand at the top of those figures in the end)